Traditionally, containers used for the storage of products for human consumption were made of glass. Typical desirable glass characteristics include transparency, indeformability and perfect label fixation. Nevertheless, because glass is fragile, easily breakable and heavy, it has become cost prohibitive, due to the high number of bottle breaks during handling. Moreover, as a result of breakage preventive measures and weight, the transportation expenses associated with glass greatly increases the cost of the product.
Numerous commodities previously supplied in glass containers are now being supplied in plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Manufacturers currently supply PET containers for various liquid commodities, such as beverages. Often these liquid products, such as juices and isotonics, are filled into the containers while the liquid product is at an elevated temperature, typically 68° C.–96° C. (155° F.–205° F.) and usually about 85° C. (185° F.). When packaged in this manner, the hot temperature of the liquid commodity is used to sterilize the container at the time of filling. This process is known as “hot filling”. The containers designed to withstand the process are known as “hot fill” or “heat set” containers.
The use of blow molded plastic containers for packaging hot fill beverages is well known. However, a container that is used for hot fill applications is subject to additional mechanical stresses on the container that result in the container being more likely to fail during storage or handling. For example, it has been found that the thin sidewalls of the container deform or collapse as the container is being filled with hot fluids. In addition, the rigidity of the container decreases immediately after the hot fill liquid is introduced into the container. After being hot filled, the heat set containers are capped and allowed to reside at generally about the filling temperature for approximately five (5) minutes. The container, along with the product, is then actively cooled so that the filled container may be transferred to labeling, packaging and shipping operations. As the liquid cools, it shrinks in volume. Thus, upon cooling, the volume of the liquid in the container is reduced. This product shrinkage phenomenon results in the creation of a negative pressure or vacuum within the container. Generally, this negative pressure or vacuum within the container ranges from 1–300 mm Hg less than atmospheric pressure (i.e., 759 mm Hg–460 mm Hg). If not controlled or otherwise accommodated, these negative pressures or vacuums result in deformation of the container which leads to either an aesthetically unacceptable container or one which is unstable. The container must be able to withstand such changes in pressure without failure.
Hot filling is an acceptable process for commodities having a high acid content. Non-high acid content commodities, however, must be processed in a different manner. Nonetheless, manufacturers and fillers of non-high acid content commodities desire to supply their commodities in PET containers as well.
For non-high acid content commodities, pasteurization and retort are the preferred sterilization process. Pasteurization and retort both present an enormous challenge for manufactures of PET containers in that heat set containers usually cannot withstand the temperature and time demands required for pasteurization and retort.
Pasteurization and retort are both processes for cooking or sterilizing the contents of a container after it has been filled. Both processes include the heating of the contents of the container to a specified temperature, usually above about 70° C. (about 155° F.), for a specified length of time (20–60 minutes). Retort differs from pasteurization in that higher temperatures are used, as is an application of pressure externally to the container. The pressure applied externally to the container is necessary because a hot water bath is often used and the overpressure keeps the water, as well as the liquid in the contents of the container, in liquid form, above their respective boiling point temperatures.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity is related to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The percentage of crystallinity is characterized as a volume fraction by the equation:
      %    ⁢                  ⁢    Crystallinity    =                    ρ        -                  ρ          a                                      ρ          c                -                  ρ          a                      ×    100  where ρ is the density of the PET material; ρa is the density of pure amorphous PET material (1.333 g/cc); and ρc is the density of pure crystalline PET material (1.455 g/cc).
The crystallinity of a PET container can be increased by mechanical processing and by thermal processing. Mechanical processing involves orienting the amorphous material to achieve strain hardening. Such mechanical processing commonly involves stretching a PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what is known as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having about 20% crystallinity in the container's sidewall.
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque, and thus, generally undesirable. Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity for those portions of the container having biaxial molecular orientation. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heated to a temperature of about 120° C.–130° C. (about 248° F.–266° F.), and holding the blown container against the heated mold for about three (3) seconds. Manufacturers of PET juice bottles, which must be hot filled at about 85° C. (185° F.), currently use heat setting to produce PET bottles having an overall container crystallinity in the range of 25–30%.
Due to the relative high cost of PET material, even slight increases in the weight of the material of the container will result in an excessive increase in its cost, making it less competitive in relation to the glass bottle, thereby resulting in the infeasibility of such a solution to the problem. Additionally, in many instances, container weight is correlated to the amount of the final vacuum present in the container after this fill, cap and cool down procedure. In order to reduce container weight, i.e., “lightweight” the container, thus providing a significant cost savings from a material standpoint, the amount of the final vacuum must be reduced. Typically, the amount of the final vacuum can be reduced through various processing options such as the use of nitrogen dosing technology or reduce fill temperatures. One drawback with the use of nitrogen dosing technology however is that the maximum line speeds achievable with the current technology is limited to roughly 200 containers per minute. Such slower line speeds are seldom acceptable. Additionally, the dosing consistency is not yet at a technological level to achieve efficient operations. Reducing fill temperatures limits the type of commodity capable of being used and thus is equally disadvantageous.
The above described negative pressure or vacuum within the container has typically been accommodated by the incorporation of structures in the sidewall of the container. These structures are commonly known as vacuum panels. Traditionally, these paneled areas have been semi-rigid by design, unable to accommodate the high levels of negative pressure or vacuum currently generated, particularly in lightweight containers. Currently, hot fill containers typically exclusively include substantially rectangular vacuum panels that are designed to collapse inwardly after the container has been filled with hot product. These rectangular vacuum panels are designed so that as product cools, they will deform and move inwardly. While commercially successful, the inward flexing of the rectangular panels caused by the hot fill vacuum creates high stress points at the top and bottom edges of the vacuum panels, especially at the upper and lower corners of the panels. These stress points weaken the portions of the sidewall near the edges of the panels, allowing the sidewall to collapse inwardly during handling of the container or when containers are stacked together.
One way to eliminate the concerns related to the above mentioned stress points is to increase the thickness of the container's sidewall. Such an increase also increases the material cost for the container and the weight of the container, both of which are unacceptable. While other such methods have worked satisfactorily to some extent, none have significantly increased to top load strength capabilities.
As exhibited from the above discussion, the sidewall portion of the container has been given considerable attention in the effort to control the mechanical stresses imposed on the container as a result of the hot-filling process. Little or no consideration has been given to the upper portion of the container, including the waist region of the container.
Containers subjected to the above-described hot filling procedure have exhibited a somewhat limited ability to withstand top loading during filling, capping, labeling and stacking for transporting or storage operations. As a result of the decreased container rigidity immediately after filling and cooling, even heat set containers are less able to resist loads imparted through the top or upper portion of the container, such as when the containers are stacked one upon another for storage and shipping (as is readily understood, it is important to be able to stack containers so as to maximize the use of shipping space). Similar top loads are imparted to the container when it is dropped and lands on the upper portion or mouth of the container. As a result of this type of top loading, the container can become deformed and undesirable to the consumer. A solution to these types of problems is critical as it would decrease the likelihood of a container's top or shoulder being deformed or crushed, as well as inhibiting ovalization in this area.
Thus, there is a need for an improved container which is designed to distort inwardly in a controlled manner under the negative pressure or vacuum which results from hot filling so as to accommodate these negative pressures or vacuums and eliminate undesirable deformation in the container yet which allows for lightweighting, accommodates higher fill temperatures, exhibits enhanced top load strength capabilities and improved empty container packout.
With the foregoing in mind, an object of the present invention is to provide novel hot fillable, lightweight plastic containers which have vacuum absorption panels that flex during hot filling, capping and cooling; which are resistant to unwanted distortion; and which absorb a majority of the negative pressure or vacuum applied to the container.
It is another object of the present invention to provide a hot filled, blow molded, lightweight plastic container which provides improved, increased top loading structural integrity.
It is also an object of the present invention to provide a lightweight container having an upper portion which includes structural characteristics that provide the container with an enhanced top load strength capability and improved empty container packout.
In function of the above mentioned qualities, associated with its transparency, the proposed lightweight container is an extremely inexpensive and efficient means for the container user to promote its product, thus contributing to reinforce the good image of its company in the market. It is therefore an object of this invention to provide such a container.